What kind of evidence could be found in the fossil record (or anywhere else)
that would prove whether or not some dinosaurs were warm-blooded?

Thomas E. Williamson of
the New
Mexico Museum of Natural History and Science is studying dinosaur fossils to
answer this question. Here is his response:

"As yet, there is probably no evidence that would definitively prove whether
or not some dinosaurs were warm-blooded. Scientists have explored numerous
lines of evidence to try to answer this question.

"There is a clear difference in bone structure between modern cold-blooded
and warm-blooded animals. Warm-blooded animals tend to have highly
vascularized bone tissue. Cold-blooded animals, on the other hand, have
relatively dense bone, sometimes even showing annual growth rings. Dinosaurs
tend to have highly vascularized bone early in life and then develop dense
bone with growth rings as they reach maturity. New evidence suggests that
the different bone types are more related to growth rates than to warm- or
cold-bloodedness.

"In modern animal communities, there are far fewer warm-blooded predators
relative to prey than cold-blooded predators. This is because warm-blooded
animals tend to eat far more than cold-blooded animals and so a given amount
of prey will support far fewer warm-blooded predators. Unfortunately, it is
very difficult to determine the actual relative abundance of predator versus
prey from the fossils because the fossil record does not always preserve an
accurate representation of the original animal community.

"Dinosaurs were able to survive even at high latitudes. For example,
dinosaur fossils have been found in Alaska and Canada well above the Arctic
Circle. During the time that these animals lived, however, the climate was
generally much warmer than today's. It is also possible that these animals
migrated into these areas during the warm summer months and retreated to
lower latitudes ahead of the cold and long polar nights.

"Modern warm-blooded animals tend to have more erect postures than
cold-blooded animals. Most dinosaurs have erect postures and therefore it
has been suggested that this indicates that they had high activity levels
and were warm-blooded. Along these lines, scientists have looked indirectly
at the potential blood pressures of dinosaurs; warm-blooded animals tend to
have relatively high blood pressures. Blood pressure can be estimated by
looking at the vertical distance between the head and the heart. For some
sauropod
dinosaurs, this estimated blood pressure is very high indeed. Brachiosaurus
would have a blood pressure of about 500 millimeters of mercury. This figure
is about five times higher than that of a human. On the other hand, other
dinosaurs--such as ceratopsians--would
have a very low blood pressure, closer to that of living reptiles, based on
this analysis.

"Modern warm-blooded animals have relatively larger brains than living
cold-blooded animals. It is thought that large brains are needed to
coordinate active, highly energetic animals. Most dinosaurs have very small
brains relative to their body size. In fact, their brains closely resemble
those of modern reptiles. A few small, predatory dinosaurs have relative
brain sizes that are comparable with those of some living birds, such as
ostriches, however.

"There is evidence from dinosaur trackways, mass accumulations of certain
dinosaur fossils, and nesting sites that dinosaurs were social animals. Some
have argued that such sophisticated behavior is suggestive of
warm-bloodedness.

"Birds are warm-blooded and probably evolved from a group of meat-eating
dinosaurs. Therefore, it has been argued, their dinosaur ancestors were also
warm-blooded. But recent study of the bone structure of some of the earliest
birds has revealed that it resembles that of modern cold-blooded reptiles,
suggesting that the first birds were cold-blooded and that warm-bloodedness
developed later.

"More recently, some researchers have looked for respiratory turbinates in
the nasal passages of dinosaurs. Respiratory turbinates are fine,
scroll-shaped bones found in the noses of most modern warm-blooded animals
(mammals and birds). These structures are believed to function as
moisture-recovery organs, recapturing water from warm and moist exhaled air.
Without these structures, many warm-blooded animals would quickly dehydrate,
especially in dry climates. So far, no respiratory turbinate structures have
been found in dinosaurs. It has been argued, however, that many dinosaurs
lived in warm and moist environments where water conservation may not have
been important.

"Dinosaurs were a very long-lived and diverse group of animals. There is as
yet no widely accepted definitive answer as to whether or not they were
warm-blooded. It is quite possible that dinosaurs had a metabolism that is
different from that of living animals. Indeed, the large size attained by
many dinosaurs may have led to what has been called 'inertial homeothermy'
or 'gigantothermy.' That is, the large bulk of the animals would have
allowed them to maintain a fairly constant temperature even without a high
metabolism. Certainly, dinosaurs were active and, in some cases, social
animals that successfully competed with warm-blooded mammals during their
160-million-year-long reign on the earth."

"It is not yet known whether there will ever be 'definitive proof' of the
metabolic status of dinosaurs. At this time, researchers must use multiple
lines of evidence to infer dinosaur energetics. Also, the physiological
alternatives are themselves complex, and cannot be adequately described by
simple terms such as warm or cold blooded. Some or all dinosaurs may have
had high aerobic exercise capacity like birds and mammals (tachyaerobic), or
low aerobic exercise capacity like reptiles (bradyaerobic). Dinosaurs may
have had high resting metabolic rates like birds and mammals, or low resting
rates like reptiles. How well dinosaurs could thermoregulate and control
body temperature is yet another unanswered question.

"Large hips suggest that most dinosaurs had large, aerobically capable leg
muscles like those of big-hipped birds and mammals, rather than small leg
muscles like those of bradyaerobic reptiles. Many dinosaur trackways show
that dinosaurs walked as fast as do birds and mammals, and much more rapidly
than can be sustained by reptiles. Predatory dinosaurs appear to have
developed birdlike respiratory systems able to take in large amounts of
oxygen. The herbivorous ornithopod dinosaurs may have had a mammallike
diaphragm for efficient respiration. The earliest and most primitive
dinosaurs, however, had smaller hips and less advanced respiratory systems,
suggesting their aerobic capacity was lower than in other dinosaurs, birds
and mammals.

"From the fossil evidence, at least some dinosaurs appear to have grown more
rapidly than any wild reptile, suggesting the juveniles were very
aerobically active and able to seek out large amounts of food.

"The great size and especially the towering height of the sauropod dinosaurs
may have required extra-high-pressure, oversized hearts that consumed much
more oxygen than is observed in reptiles.

"The evidence for resting metabolic rates is more ambiguous. Some
continental birds and mammals, for example, have poorly developed
respiratory turbinates or nasal passages no larger than those of dinosaurs
of similar size. Some birds and mammals have very high aerobic
exercise-to-resting ratios, but no tachyaerobic animal has a reptilian
resting metabolic rate. If most dinosaurs were tachyaerobic, then they
probably had high resting metabolic rates as well.

"It is even less clear whether dinosaurs thermoregulated as effectively as
do birds and mammals. Growing dinosaurs laid down rings inside their bones
more often than birds and mammals, and this hints they did not regulate body
temperature to the same degree.

"In conclusion, the evidence that most dinosaurs had high aerobic exercise
capacity like birds and mammals is very good. It is probable that their
resting metabolic rates were higher than in reptiles. It is possible that
many dinosaurs did not maintain a constant body temperature. The energetics
of dinosaurs almost certainly were not reptilian, but just how closely they
approached the avian-mammalian condition is not yet certain."

Philip N. Froelich, director of the School of Earth and Atmospheric Sciences
at the Georgia Institute of Technology,
offers yet another view:

"One possibility would be to look at oxygen isotopic abundances in the
annual growth bands of teeth or bones of high latitude (and hence,
presumably, seasonally influenced) terrestrial dinosaurs. The ratio of
oxygen isotopes depends on temperature. An absence of seasonal variations in
oxygen 18--a heavy version of the common oxygen 16 atom--would strongly
suggest that the animals maintained a constant internal temperatures.

"Such a finding would not, however, constitute 'proof' that dinosaurs were
warm blooded, as there are external mechanisms that cold-blooded animals
employ to regulate body temperatures and thereby influence metabolic rates.
Proof would have to come from discovery of intact dinosaur remains in which
the soft tissue had not been replaced or altered, and from which the
biomolecules responsible for thermoregulation could be extracted, identified
and characterized. Such a proof is very, very unlikely, as it would require
an almost impossible level of preservation over 65 million years, plus the
advent of biotechnology that does not (yet) exist to elucidate the
thermoregulation biochemistry of an extinct species.

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